A goal of ventilatory support is to reduce the internal work of breathing in subjects with disease of the lungs or chest wall. Typical mechanical ventilators deliver a varying pressure to the airway, for example, via a face mask or nose mask. In a spontaneously breathing patient, a fundamental problem that must be solved is the synchronization of the delivered pressure with the patient's spontaneous efforts. For example, in a typical bilevel ventilator, the mask pressure is switched to a higher pressure, for example, 20 cmH2O, at the moment of detection of patient inspiratory airflow, and a lower pressure, for example, 5 cmH2O, at the moment of cessation of patient inspiratory airflow. There are at least four problems with the use of patient respiratory airflow to trigger the device between the higher and lower pressures:
(1) large and varying leaks cause a discrepancy between patient respiratory airflow (the desired quantity) and mask flow (the measured quantity);
(2) dynamic airway compression and intrinsic positive pressure cause a mis-match between effort (the truly fundamental quantity) and airflow (the measured quantity);
(3) cardiac emptying produces airflow that can be confused with respiratory airflow; and
(4) in patients with high airway resistance causing long respiratory system filling and emptying time constants, inspiratory airflow can continue after cessation of inspiratory effort, and expiratory airflow can continue despite recommencement of inspiratory effort.
There are several known ways to find a more direct measure of patient respiratory effort than is afforded by respiratory airflow. One method is to invasively measure intrathoracic pressure, for example, by placing a pressure sensing catheter in the oesophagus. Increasing respiratory effort produces increasing subatmospheric pressure at the sensor. However, this method is too invasive for general use. Another method is to place respiratory movement sensors around the chest wall, typically one around the thorax and one around the abdomen. Suitable sensors use either inductance pneumography, although reasonable signals can be obtained using strain gauges, magnetometers, graphite-in-rubber bands, etc. Unfortunately, none of these methods truly measures respiratory effort as opposed to the resulting chest wall movement, and if the airway is partially or completely occluded, then the signal can greatly diminish in amplitude even though the respiratory effort has actually increased. Also, changes in mechanics with body position or sleep state make these prior art sensors unreliable.
A third method is to use a diaphragm electromyogram as a measure of spontaneous effort, but this is extremely technically demanding, unsuitable for long term use, and difficult to process due to cardiac artifact. A fourth method, taught by McWilliams in U.S. Pat. No. 5,513,631, is to monitor movement of the nostrils, which flare some moments prior to commencement of spontaneous inspiration under some circumstances. Unfortunately, this may not occur in all sleep states, or under conditions of relatively normal respiratory drive, making it less useful as the condition of concern becomes partially treated.
Another fundamental problem is to determine how much support to give, for example, whether to vary the pressure by 15 cmH2O as in the above example, or a smaller or larger value. Various known methods attempt to tailor the degree of support to suit the patient. One method is to deliver a fixed degree of support using a bilevel ventilator, and to attempt to tailor that degree of support to be best on average for the patient, by aiming to strike a balance between optimizing arterial oxygenation or carbon dioxide level versus providing comfort to the patient. An advantage is that patients are free to take larger or smaller breaths, and at varying rates, which helps with comfort, but a disadvantage is the inability to provide suitable support under varying conditions. Another essentially opposite method is to provide a fixed volume of air per breath (volume cycled ventilator) or a fixed volume per minute (servo ventilator), which is more effective but less comfortable. A pressure support servo ventilator, for example, as taught in PCT Application 97/00631 of Berthon-Jones entitled “Assisted Ventilation to Match Patient Respiratory Need”, combines the comfort advantages of a pressure support ventilator with the blood gas optimizing advantages of a servo ventilator (as well as other advantages related to patient-machine synchronization). Still another method, proportional assist ventilation, is described in Younes U.S. Pat. No. 5,107,830. Proportional assist ventilation measures patient respiratory airflow, and provides support for the resistive component of work of breathing proportional to the respiratory airflow, and support for the elastic component proportional to the integral of respiratory airflow. All the above methods still have varying degrees of problems associated with the difficulties of measurement of respiratory airflow under conditions of high leak.
A ventilator, in general, comprises three parts:
(1) a respiratory effort sensor, or a sensor of a surrogate of respiratory effort,
(2) a source of breathable gas at a controllable pressure delivered via a mask or similar interface to a patient's airway, and
(3) a control that modulates the mask pressure so as to reduce the respiratory effort signal.
Generally, the respiratory effort signal is inferred from a surrogate such as mask pressure or flow. Both are unreliable indicators of effort in the presence of leaks, particularly changing leaks, which are particularly ubiquitous during noninvasive ventilation. They are also unreliable indicators in the presence of intrinsic PEEP (Positive End Expiratory Pressure), which is common in severe lung disease, because effort can precede flow by an appreciable period. They are also unreliable in the presence of a long respiratory time constant, as is universal in obstructive lung disease, because machine-induced pressure changes lead to long-lasting flow changes even when the patient has ceased inspiratory effort. More direct measures of respiratory effort known to be useable for the control of ventilators are either invasive or unreliable or both.